Pyrochemical reprocessing method for spent nuclear fuel and induction heating system to be used in pyrochemical reprocessing method

ABSTRACT

This invention is provided for improvement of corrosion-resistant property of a crucible and for promotion of safety in a pyrochemical reprocessing method for the spent nuclear fuel. The spent nuclear fuel is dissolved in a molten salt placed in the crucible. In a pyrochemical reprocessing method, the nuclear fuel is deposited, and the crucible ( 2 ) is heated by induction heating. Cooling media ( 5, 6 ) are supplied to cool down, and a molten salt layer ( 7 ) is maintained by keeping balance between the heating and the cooling, and a solidified salt layer ( 8 ) is formed on inner wall surface of the crucible.

BACKGROUND OF THE INVENTION

The present invention relates to a pyrochemical reprocessing method ofspent nuclear fuel used in atomic reactor and to an induction heatingsystem used in the pyrochemical reprocessing method.

Studies have been performed, both domestic and abroad, on recyclingmethod to improve economic feasibility of nuclear fuel cycle, by whichthe spent nuclear fuel is reprocessed by pyrochemical reprocessingmethod using molten salt and uranium or plutonium and the spent fuel iscollected and recycled.

In the pyrochemical reprocessing of oxide fuel, spent nuclear fuel isdissolved in a molten salt, and oxides of uranium or plutonium ingranular state are deposited and are collected by electrolysis.Principal processes are as follows:

Chlorination dissolving process of spent fuel:

UO₂+Cl₂→UO₂Cl₂

PuO₂+C+Cl₂→PuCl₄+CO₂

Electrolysis and collecting process of uranium oxide (cathode):

UO₂Cl₂→UO₂+Cl₂

Precipitating and collecting process of plutonium oxide:

PuCl₄+O₂→PuO₂+2Cl₂

The crucible used for reprocessing is made of pyro-graphite, and itexerts action as an anode in the electrolysis and depositing process.

In the pyrochemical reprocessing method for metal fuel, spent nuclearfuel is dissolved in a molten salt. Metal uranium or metal plutonium aredeposited and are collected by electrolysis. Principal processes are asfollows:

Dissolving processing of spent fuel:

U→U³⁺+3e⁻; Pu→Pu³⁺+3e⁻

Electrolysis and collecting process of metal, uranium (solid cathode):

U³⁺+3e⁻→U

Electrolysis and collecting process of uranium oxides or plutoniumoxides (liquid cathode):

U³⁺+3e⁻→U; Pu³⁺+3e⁻→Pu

(1) In the conventional heating method used for the melting of the spentnuclear fuel in the molten salt, the crucible is directly heated. As aresult, the crucible acts as heat transmitting surface, and it turns toa corrosion environment at higher temperature than the meltingtemperature of the salt.

(2) The crucibles are directly exposed to chlorine gas, oxygen gas, etc.used in the pyrochemical reprocessing, and they make severe corrosionenvironment.

(3) Compared with induction current used in the conventional metalmelting method, higher frequency is needed due to the difference inelectric conductivity.

(4) It is generally practiced to adopt water-cooling method in order tocool down the crucible used for induction heating. However, when wateris brought into contact with the molten salt by any chance, explosionmay occur.

(5) In the conventional heating method, stirring means are required toevenly maintain temperature distribution of the molten salt, and thisrequires the designing of the entire system in more complicatedstructure.

(6) In the resistance heating method, the molten salt is evenly moltenand evenly stirred up, and a cylindrical crucible must be generallyadopted.

SUMMARY OF THE INVENTION

To solve the above problems, it is an object of the present invention toprovide a method and a system, by which it is possible to improvecorrosion-resistant property of the crucible and to contribute to thesafety in the pyrochemical reprocessing method.

To attain the above object, the present invention provides apyrochemical reprocessing method for spent nuclear fuel for melting thespent nuclear fuel in a molten salt placed in a crucible and bydepositing the nuclear fuel, whereby the crucible is heated by inductionheating, a cooling medium is supplied to cool down, and a molten saltlayer is maintained by keeping balance between the heating and thecooling, and a solidified salt layer is formed on inner surface of thecrucible.

Further, the present invention provides an induction heating system tobe used in a pyrochemical reprocessing method for melting a spentnuclear fuel in a molten salt placed in a crucible and for depositingthe nuclear fuel, wherein said induction heating system comprises meansfor induction heating, and cooling means for cooling by supplying acooling medium to the crucible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents schematical drawings each showing an induction heatingsystem;

FIG. 2 shows an example of an arrangement of the induction heatingsystem;

FIG. 3 represents drawings each showing an example of a crucible typeheater designed in cylindrical shape;

FIG. 4 represents drawings each showing an example of a crucible typeheater designed in annular shape;

FIG. 5 represents drawings each showing an example of a crucible typeheater designed in planar (rectangular) shape;

FIG. 6 represents drawings to show the crucible and induction heatingcoil integrated with each other;

FIG. 7 is a diagram showing an example of formation of a solidifiedlayer of salt; and

FIG. 8 shows free surface shape of a molten salt and distribution ofelectromagnetic force.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Description will be given below on embodiments of the present inventionreferring to the drawings.

FIG. 1 represents schematical drawings each showing an arrangement of aninduction heating system. FIG. 1 (a) is a partially cutaway perspectiveview, and FIG. 1 (b) is a cross-sectional view.

A crucible 2 divided by a segment 1 is placed in a high frequencyinduction coil 3, and magnetic field is applied directly on salt, whichlies in molten state in the crucible. In melting the salt, an auxiliaryheating member 4 made of conductive material is added as necessary. Thecrucible 2 is designed in such structure that its inner space is cooleddown by using cooling media 5 and 6. By keeping balance between theheating power supplied from an AC power source 13 to the high frequencyinduction coil 3 and the cooling, a solidified salt layer 8 is formed inthe boundary between the crucible 2 and the molten salt 7.

As the cooling media 5 and 6, fluid (liquid or gas) is used. To cooldown the crucible, water-cooling method is generally used. However, whenwater is brought into direct contact with the molten salt by any chance,explosion may occur. To avoid the explosion, a cooling medium having aboiling point higher than the temperature of the molten salt or lowerthan operating temperature of the cooling medium is used. As the coolingmedium having boiling point higher than the temperature of the moltensalt, potassium (boiling point: 765.5° C.), sodium (boiling point:881.1° C.), etc. may be used. As the cooling medium having a boilingpoint lower than the operating temperature, nitrogen (boiling point:−195.8° C.), helium (boiling point: −268.9° C.), etc. may be used.

One of the features of the present invention is that a solidified layerof salt is formed in the boundary between the crucible and the moltensalt by keeping balance between electric power and cooling using aninduction heating system. In the figure, reference numeral 9 is a gasblow pipe for blowing a process gas such as chlorine gas or oxygen gas.Numeral 10 denotes an off-gas exhaust pipe. A predetermined voltage isapplied from a DC power source 14 to a cathode 11 and an anode 12, andelectrolysis and recovery of the spent fuel are performed at the cathode11.

FIG. 2 shows an arrangement of the induction heating system.

The crucible 2 is placed in the high frequency induction coil 3. To thiscoil, high frequency power is supplied from a high frequency generatorof the AC power source 13, and the salt in the crucible is molten. Thecooling medium is supplied from a cooling medium re-cooling andcirculating system 15 to the crucible 2. A solidified layer of salt isformed in the boundary between the crucible and the molten saltdepending on the balance between the high frequency electric power forheating and the cooling.

For cooling the crucible, liquid or gas is used as described above. Thesalt used is a salt such as CsCl, NaCl, KCl, etc. or a mixed saltcontaining these salts. Melting point of KCl—NaCl mined salt of equalmole concentration is about 660° C. as an example of the salt used. Whenthis salt is molten, surface temperature in the crucible is about 50° C.as an example of one test, and temperature gradient is generated in thesolidified salt layer 8.

As described above, temperature gradient occurs when the solidifiedlayer is formed. Thus, it is possible to decrease crucible temperaturewithout cooling the molten salt except the solidified layer (whilemaintaining the molten state), and corrosion environment in the cruciblecan be alleviated. Also, by forming the solidified layer, it is possibleto alleviate or avoid the condition where inner surface of the crucibleis directly exposed to chlorine gas or oxygen gas. As a result, servicelife of the crucible material, i.e. the material of the heating system,can be extended.

Next, various shapes of the crucible are shown in FIG. 3 to FIG. 5. Inthe following, no description will be given on the cooling system, whilethe solidified layer is formed by keeping balance between the heatingelectric power and the cooling in the same manner as described above.

FIG. 3 shows an example where the crucible is designed in cylindricalshape. FIG. 3 (a) is a cross-sectional view, and FIG. 3 (b) is avertical sectional view.

A crucible 22 in cylindrical shape is placed inside a cylindrical coil21. A solidified salt layer 23 is formed on inner wall surface of thecrucible, while a molten salt layer 24 is maintained inside. To promotetemperature increase, auxiliary heating members 25 made of conductivematerial are arranged inside the crucible.

FIG. 4 shows an example of a crucible in annular shape. FIG. 4 (a) is across-sectional view, and FIG. 4 (b) is a vertical sectional view.

An outer annular crucible 33 and an inner annular crucible 34 arearranged between an outer annular coil 31 and an inner annular coil 32,and a molten salt is placed between walls of the two annular crucibles.A solidified salt layer 35 is formed on inner wall surfaces of the outerand inner annular crucibles, and a molten salt layer 36 is maintained.To promote temperature increase, auxiliary heating members 37 made ofconductive material are placed in the crucible.

FIG. 5 shows an example of the crucible designed in planar (rectangular)shape. FIG. 5 (a) is a cross-sectional view, and FIG. 5 (b) is avertical sectional view.

A rectangular crucible 42 is arranged inside a rectangular coil 41. Asolidified salt layer 43 is formed on inner wall surface of thecrucible, while a molten salt layer 44 is maintained. To promotetemperature increase, auxiliary heating members 45 made of conductivematerial are arranged in the crucible.

FIG. 6 shows an integrated example of a crucible and an inductionheating coil. FIG. 6 (a) is a cross-sectional view, FIG. 6 (b) is avertical sectional view, and FIG. 6 (c) is a partially enlarged view.

In this embodiment, the crucible and the induction heating coil areintegrated to increase the heating efficiency. A coating material istreated in gaps and on inner surface of a rectangular coil 51, and thisis used as crucible 52. A solidified salt layer 53 is formed on innerwall surface of the crucible, while a molten salt layer 54 is maintainedinside. To promote temperature increase, auxiliary heating members 55made of conductive material are arranged in the crucible. As for theshape of the crucible coil, the shapes as shown in FIG. 3 to FIG. 5 orvariations of these shapes, e.g. rectangular shape in dual structure asshown in FIG. 4, may be adopted.

According to the present invention, the following effects can beachieved:

(1) It is possible to decrease the surface temperature inside thecrucible.

Table 1 summarizes a comparative example of surface temperature of innerwall of the crucible under direct heating (e.g. resistance heating) andinduction heating when KCl—NaCl mined salt of equal concentration isused.

TABLE 1 Heating mode Direct heating Induction heating Surface Higherthan 660° C. 50° C. temperature in crucible

As shown in Table 1, compared with the conventional direct heatingmethod, it is possible for the induction heating method to cool down thesurface in the crucible while maintaining the molten salt layer by thecooling system. As a result, corrosion environment like the conventionalmethod can be extensively improved, and service life of the materialscan be extended.

(2) Immediate contact to the crucible material of the corrosive elementsis prevented according to the solidified salt layer

An example of formation of the solidified salt layer is shown in FIG. 7.In FIG. 7, a distance (mm) from the wall surface of the crucible isrepresented on the axis of abscissa, and temperature (° C.) is shown onthe axis of ordinate. From this diagram, it is evident that boundarybetween the solidified layer and the molten layer lies near the portion8-9 mm from the wall surface of the crucible. By the presence of thesolidified layer of salt, it is possible to avoid direct contact betweenthe crucible wall surface and corrosive elements such as chlorine gas,or oxygen gas. As a result, corrosive conditions as seen in theconventional method can be extensively improved, and surface life of thematerials can be extended.

(3) Promotion of temperature increase by the use of auxiliary heatingmembers

When salt is molten, it is necessary to have higher frequency because ofthe difference of electric conductivity compared with induction currentused in the conventional metal melting. By arranging auxiliary heatingmembers made of conductive material, it is possible to promote andaccelerate temperature increase compared with the temperature increaseof salt by the induction current as used in the conventional method formetal melting.

(4) Explosion caused by the contact of the molten salt with the coolingmedium can be prevented by the adoption of a cooling medium exceptwater.

To cool down a crucible used for induction heating, it is generallypracticed to adopt water-cooling system. However, explosion may occurwhen the molten salt is brought into direct contact with water. Byadopting a cooling system using a cooling medium except water, it ispossible to avoid the danger such as explosion.

(5) Stirring effect of electromagnetic force generated by inductionheating

FIG. 8 represents an example, which shows by numerical analysis whetherstirring effect is caused or not by generation of magnetic field due tothe induction heating. The figure shows free surface configuration ofthe molten salt and distribution of electromagnetic force (distributionof electric power force and magnetic force) from the crucible center tocrucible wall surface. The electromagnetic force is concentrated at thecloser part to the crucible wall and gives higher effect. Due tounevenness of the electromagnetic force, electromagnetic stirringoccurs, and this gives stirring effect.

(6) Diverse crucible shapes

In the resistance heating method, the molten salt is evenly molten andevenly stirred up, and a crucible in cylindrical shape is generallyadopted. Because even melting and even stirring can be expected due tothe stirring effect as described in (5) above, diverse crucible shapesas shown in FIG. 3 to FIG. 5 and other variations based on these shapescan be adopted.

(7) Application to other systems and devices in addition to a meltingcrucible

By utilizing non-contact heating, which is one of the features of theinduction heating method, it is possible to adopt the present method asthe heating system in such applications as cathode processor ordistillation cleaning of the used salt.

What is claimed is:
 1. An induction heating system to be used in apyrochemical reprocessing method for melting a spent nuclear fuel in amolten salt placed in a crucible and for depositing the nuclear fuel,wherein said induction heating system comprises means for inductionheating, cooling means for cooling by supplying a cooling medium to thecrucible, and auxiliary heating means for auxiliary heating, comprisinga heating member arranged in the crucible to promote temperatureincrease of the molten salt.
 2. An induction heating system according toclaim 1, wherein said crucible is formed in a shape of one of the groupconsisting of a cylindrical shape, an annular shape, a planar shape or ashape formed by combining these shapes.
 3. An induction heating systemaccording to claim 1, wherein a fluid except water is used as thecooling medium.
 4. An induction heating system according to claim 1,wherein said crucible is pyro-graphite.
 5. An induction heating systemaccording to claim 4, wherein said pyro-graphite crucible is an anode inan electrolysis and depositing process.